TIMED is a NASA exploratory mission, managed and operated by JHU/APL (Johns Hopkins University/Applied Physics Laboratory) for NASA. TIMED is the first mission in NASA's STP (Solar Terrestrial Probes) program. The primary objective is to investigate and understand the energetics of the Mesosphere and Lower-Thermosphere/Ionosphere (MLTI), the region in the Earth's atmosphere from about 60 to 180 km in altitude. The measurements of TIMED will provide data defining the basic states of the MLTI region and its thermal balance. Specific missions goals are: 1)2)3)4)5)6)7)8)

• To determine the temperature, density and solar wind structure in the MLTI region, including the seasonal and latitudinal variations

• To determine the relative importance of the various radiative, chemical, electrodynamical, and dynamical sources or sinks of energy for an understanding of the thermal structure of the MLTI.

Areas of applications: The MLTI (Mesosphere and Lower-Thermosphere/Ionosphere) is a very poorly understood region of the atmosphere. The results of the TIMED mission will enable the scientific community to establish the first quantitative MLTI baseline and will serve as a basis for future investigations.

The ADCS (Attitude Determination and Control Subsystem) employs its own MIL-STD-1553B data bus. This decouples the ADCS from the rest of the S/C subsystems, permitting for the easy implementation of autonomy and safing algorithms. A complete loss of attitude due to a failure of the attitude 1553 bus controller can be recognized by the C&DH processor, which can switch to the redundant AIU (Attitude Interface Unit).

During normal operations, TIMED is required to maintain a nadir-pointing attitude with the +z axis in the direction of the Earth's geocentric center, the +y axis in the direction of orbit normal, and the +x axis generally in the ram or wake direction.

The Sun beta-angle, defined as the angle between the Sun and orbit plane, dictates the x-axis direction as follows. SABER contains a thermal cooler for instrument temperature regulation and requires the spacecraft to avoid pointing the +y vehicle axis toward the Sun. Orbit precession thus necessitates a π-radian rotation, called the "yaw maneuver," about the z-axis approximately once every sixty days. In addition to the nadir pointing science mode, a power and thermally safe attitude mode exists in which the –y side of the spacecraft is pointed at the Sun and the solar arrays are rotated to face the Sun (Ref. 37).

Figure 3: The spacecraft block diagram (image credit: JHU/APL)

IEM (Integrated Electronics Module):

TIMED has been designed with a significant amount of onboard autonomy, as it is run with a low-cost mission operations concept. The spacecraft with redundant subsystems features an IEM that contains RF and digital subsystems in a common card cage. The cards within the IEM communicate over a PCI (Peripheral Component Interconnect) parallel data bus. Two identical IEM modules are used, both communicating to other spacecraft subsystems and the instruments over a redundant 1553 serial data bus. The IEM collects housekeeping data from other cards and subsystems through a 100 kHz twisted-pair housekeeping (I2C) bus. Three spacecraft subsystems and supporting power-conditioning electronics, all implemented on nine plug-in cards, are integrated into a single chassis to form the IEM consisting of: 11)12)13)

Legend to Figure 5: The size of IEM is 18 cm x 33 cm x 27 cm, the mass is 11.4 kg, and the peak power consumption is 53.3 W (when the downlink amplifier is on).

C&DH (Command & Data Handling): The C&DH subsystem is implemented on three cards—processor, solid-state recorder (SSR), and C&T (Command and Telemetry) interface. It also has elements on two other cards: a CCD (Critical Command Decoder) on the uplink card and a downlink formatter on the downlink card, both part of the telecommunications subsystem. The C&DH features are:

- Processor and 1553 bus controller (MIL-STD-1553B, redundant). A dual-processor-based computer system is used (Synova Mongoose-V 32-bit RISC processors with 2 MByte of RAM and 4 MByte of Flash EEPROM)

The GNS (GPS Navigation subsystem) resides on two cards. One card contains a receiver consisting of an RF downconverter, an oscillator, a synthesizer, and a 12-channel GTA (GPS tracking ASIC) chip.

RF communications: The scalable architecture of the S-band RF transceiver system is part of the IEM. The transceiver consists of two RF cards, each of size 15 cm x 22 cm. The IEM approach provides the integration of functionalities that are normally part of the system (inclusion of a real-time critical command decoder). The downlink card outputs a CCSDS-compatible downlink signal and can support data rates up to 4 Mbit/s. The downlink card also includes an on-board data framer and Reed-Solomon encoder.

The G&C (Guidance and Control) hardware architecture, consists of two star trackers, redundant 3-axis IRUs (Inertial Reference Units), redundant 3-axis magnetometers, redundant Sun sensors on each of the +y and -y sides, three electromagnetic torquer bars (EMTs) containing redundant coils, four reaction wheel assemblies, and a pair of redundant processors. One processor, the AIU (Attitude Interface Unit), interfaces the G&C system with the rest of the spacecraft and provides logic for safe mode pointing and momentum management. The other processor, the AFC (Attitude Flight Computer), provides the logic for the nadir pointing mode (Ref. 37).

- A GNS dual processor, one processor for tracking the received GPS signals, the other for producing navigation results and other functions

- GPS receiver and digital signal processing ASIC that form 12 channels for tracking GPS signals. A pre-amplifier, external to the IEM and located close to the GPS antenna, supplies the incoming GPS signals to the receiver

The GNS was designed for the hostile radiation environment of space. It is latch-up immune, has a very low SEU (Single-Event-Upset) rate and, except for the computer memory, is hardened to >300 krad (Si). The core electronics can sustain total dose radiation in excess of 1 Mrad (Si). The system has extensive command and telemetry capability and provides access to raw and intermediate data products.

The GNS receive antenna is located on the spacecraft's optical bench on the zenith-pointing surface. The pre-amplifier, consisting of pre-select filters and a low noise amplifier, is located just underneath the optical bench and generates the dominant component of the system thermal noise power. The remainder of the system, composed of an RF downconverter, a baseband signal processing subsystem, and a dual microprocessor, is located on two Stretch-SEM-E circuit boards and housed in the TIMED IEM.

Noncoherent navigation: In addition, the transceiver system cards provide the capability for highly accurate Doppler tracking, using a two-way noncoherent technique. The APL-developed technique obviates the need for coherency between the uplink's carrier tracking oscillator and the downlink carrier. In this technique, the uplink carrier signal is received and compared with the receiver's onboard reference oscillator. This operation results in a set of phase comparison counts, placed in the telemetry and transmitted to the ground. There, the ground station continues tracking the S/C as if it were a coherent transponder. The uplink and downlink error counts are compared with the telemetered phase comparison counts. Tracking accuracies to 0.1 mm/s (velocity error) are achieved.

Power conditioning electronics:

- Power from the spacecraft's switched bus (unregulated), nominally +28 VDC (Volt Direct Current), is converted to regulated and filtered voltages for the C&DH and GNS cards within the IEM and for the RIU (Remote Interface Units) external to the IEM

- Power from the spacecraft's switched and unswitched busses is converted to regulated and filtered voltages for the RF communications subsystem. 15)

The TIMED onboard Autonomy System has two basic functions: 1) to ensure fault protection and safing of the spacecraft, and 2) to perform a limited set of routine operations. The main requirements of fault protection and safing are to detect failed or improperly functioning spacecraft components and to autonomously replace them in the operational configuration with properly functioning counterparts. 16)

The IEM concept was to incorporate multiple spacecraft subsystems into a singe chassis, thereby conserving critical spacecraft resources at a reduced cost. The IEM was implemented on TIMED including C&DH, GNS, RF telecommunications, and IEM power conditioning (Ref. 13).

Launch: A launch of TIMED took place on Dec. 7, 2001 aboard a Delta-2 launcher from VAFB, CA (co-manifest with Jason-1).

Orbit: Circular orbit, altitude = 625 km; inclination = 74.1º with a720º per year nodal regression (this means the local time of the equator crossing varies from dawn to dusk in three months).

TIMED uses an operations concept called "event-based" commanding, which eliminates the need for repetitive time-tagged commands. The TIMED GNS (GPS Navigation System) enables this mode of operation. Using the GNS, which consists of a GPS receiver and orbit propagator, the spacecraft knows its position and velocity at any given time and can predict events such as terminator crossings and passages over auroral zones and ground stations.

Mission operations are performed at JHU/APL in Laurel, MD. The payload operations concept requires each PI to perform his own instrument operations from his home site. A combination of onboard GPS processing and the use of Internet move the data from the APL ground station to each investigator's home site. By separating instrument operations from all S/C system activities, the instrument teams are able to control all of the instrument modes, operations and science data return at their own choice without explicit interactions or approvals by the S/C project team. In addition, the Internet is used by each investigator to control his instrument directly (the packetized messages are integrated into the uplink command structure in an automated fashion).

TIMED mission status:

• January 4, 2019: The TIMED spacecraft is operating nominally and is currently collecting data on one of the last frontiers in Earth's atmosphere. On 7 December 2018, TIMED completed 17 years on orbit. 17)

• January 2018: SABER global infrared emission power from 5.3 µm NO and 15 µm CO2 through the end of 2017. These are slowly decreasing from the maximums reached in December 2014 as the current solar cycle approaches it's minimum. 18)

Figure 9: The SABER (Atmosphere using Broadband Emission Radiometry) instrument is one of four instruments on NASA's TIMED (Thermosphere Ionosphere Mesosphere Energetics Dynamics) satellite. The primary goal of the SABER experiment is to provide the data needed to advance our understanding of the fundamental processes governing the energetics, chemistry, dynamics, and transport in the mesosphere and lower thermosphere. SABER accomplishes this with global measurements of the atmosphere using a 10-channel broadband limb-scanning infrared radiometer covering the spectral range from 1.27 µm to 17 µm. These measurements are used to provide vertical profiles of kinetic temperature, pressure, geopotential height, volume mixing ratios for the trace species O3, CO2, H2O, [O], and [H], volume emission rates for 5.3 µm NO, 2.1 µm OH, 1.6 µm OH, and 1.27 µm O2(1Δ), cooling and heating rates for many CO2, O3, and O2 bands, and chemical heating rates for 7 important reactions (image credit: JHU/APL)

• On January 31, 2017, the SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) team members gathered at NASA/LaRC (Langley Research Center) to acknowledge and celebrate 15 years of atmospheric discovery on TIMED. "SABER, which marked 15 years of on-orbit operation on Jan. 22, 2017, has provided a never-before-seen view of the atmosphere and paved the way for new avenues of scientific study," said NASA Langley's Deputy Director Clayton Turner. "Fifteen years of SABER data has deepened our knowledge of the planet's radiation budget — the balance between Earth's incoming and outgoing energy. That's an important achievement." 19)

- The Earth's atmospheric limb, known as the MLTI (Mesosphere and Lower Thermosphere/Ionosphere) region — is home to the International Space Station and hundreds of satellites in Low Earth Orbit. It is also where the sun's energy first impacts Earth's atmosphere. Although it is the first shield from the sun's ultraviolet radiation and contains important gases such as ozone, water vapor and carbon dioxide, very little was known about this thin, outer layer between 16 -175 km in altitude. - Fifteen years ago, the MLTI was considered one of the least explored regions of Earth. But thanks to SABER on TIMED there is a new wealth of comprehensive global measurements of the MLTI.

- The SABER dataset is the first global, long-term, and continuous record of thermosphere, or upper atmospheric, emissions of nitric oxide and carbon dioxide — molecules that, in this region of the atmosphere, serve as atmospheric thermostats that send upper atmospheric heat back into space.

- One well-documented occurrence of this transfer of heat was in 2012 when over just three days, solar storms dumped enough energy in Earth's upper atmosphere to power every residence in New York City for two years. SABER data revealed that the nitric oxide and carbon dioxide in the thermosphere re-radiated 95 percent of that energy back into space. - Data from that event, as well as other more recent solar events, continue to be analyzed to determine the effect on Earth's upper atmosphere.

- James Russell III, SABER principal investigator and co-director of the Center for Atmospheric Sciences at Hampton University adds, "We broke new ground on the coupling of high and low atmosphere, and on the long-term change in carbon dioxide, water and other gases."

• December 7, 2016: Launched Dec. 7, 2001, NASA's TIMED spacecraft has spent 15 years observing the dynamics of the upper regions of Earth's atmosphere – comprising the mesosphere, thermosphere and ionosphere. The slice that TIMED studies spans altitudes of about 60 to 180 km above Earth's surface. Here, the atmosphere is just a tenuous wash of particles that reacts both to energy inputs from above – from changes in the space environment largely due to the sun – and forcing from below, including terrestrial winds. 20)

- TIMED's 15 years of data has given scientists an unprecedented perspective on changes in the upper atmosphere. The long lifespan has allowed scientists to track the upper atmosphere's response to both quick-changing conditions – like individual solar storms – throughout the sun's 11-year activity cycle, as well as longer trends, like TIMED's detection of unexpectedly fast increases in carbon dioxide in Earth's upper atmosphere.

- NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the TIMED mission for the Heliophysics Division within the Science Mission Directorate at NASA Headquarters in Washington. The Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, built and operates the spacecraft for NASA.

• October 28, 2015: NASA's TIMED mission has confirmed a surprisingly fast carbon dioxide increase in Earth's upper atmosphere, raising questions about how different layers of the atmosphere are interconnected. Even more curious — though climate models predict carbon dioxide should increase more or less equally across the globe, in its 14 years of data collection, TIMED observed a much faster increase of carbon dioxide above the Northern Hemisphere. 21)

- Understanding the way carbon dioxide moves throughout the atmosphere is key, both for making accurate climate models and for planning spacecraft flight paths. Though carbon dioxide raises temperatures near Earth's surface, it actually causes cooling in the upper atmosphere, reducing air density in these outermost reaches of the atmosphere and impacting spacecraft orbits.

- This study, published in Geophysical Research Letters on Sept. 5, 2015, uses 14 years of data from a radiometer on board the TIMED satellite, the first satellite capable of making long-term measurements of carbon dioxide concentration in the upper atmosphere when it launched in 2001 (Figure 10).

- "Before TIMED, the only measurements of carbon dioxide in the upper atmosphere were direct measurements from sounding rocket research flights and short-lived spaceborne sensors," said Jia Yue, a researcher at Hampton University in Hampton, Virginia, and lead author on the study. "But it's impossible to study long-term trends from snapshots."

- Carbon dioxide is being poured into the atmosphere by human activities, like the burning of fossil fuels and deforestation. A 5%/decade increase in carbon dioxide concentration in the lower atmosphere is confirmed by some 56 years of measurements at Earth's surface. But in the upper atmosphere, the increase in carbon dioxide concentration was observed reaching rates of 12%/ decade around 70 miles above the surface.

- Furthermore, the study team discovered that the carbon dioxide in these upper layers, long thought to follow the same patterns across the globe, is increasing faster over the Northern Hemisphere. Though the Northern Hemisphere produces much more carbon dioxide because of its greater land area and population, scientists expect the difference to become negligible at such great heights due to diffusion and mixing. "It seems clear that we don't quite understand the relationship between the lower atmosphere and the upper atmosphere," said Diego Janches, TIMED project scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "We tend to separate them into different fields—lower atmosphere is Earth science, upper atmosphere is heliophysics—but we need to understand the atmosphere as a complete system."

- This study's result also confirms a second set of data from a satellite with capabilities similar to TIMED—the Canadian Space Agency's SciSat-1, launched in 2003. An analysis of eight years of SciSat-1 data gave the first indication of the accelerated increase of carbon dioxide in the upper atmosphere. "The support between observations by TIMED and SciSat-1—which use different remote sensing techniques to detect carbon dioxide—confirms that the quick increase is a real, physical trend, and not an artifact of any instrument or data," said Yue.

- Identifying these unexpected trends was only possible because of the long lifespan of TIMED's radiometer, which is still operating well. The TIMED satellite, originally slated for a two-year mission ending in 2003, has been granted six extended missions. TIMED's current mission is set to end in 2017, but scientists are hopeful that data collection will continue.

Figure 10: Data from the SABER instrument of TIMED have confirmed a surprisingly fast carbon dioxide increase in Earth's upper atmosphere using 14 years of data from a radiometer aboard the satellite (image credit: NASA)

• Sept. 17, 2015: The TIMED spacecraft, which studies the impact of solar- and human-induced disturbances on Earth's upper atmosphere, celebrated 5,000 days of continuous data collection on Aug. 15, 2015. During that time, the spacecraft completed more than 74,000 Earth orbits and accumulated more than 9 TB of data, served by the TIMED Mission Science Data Center located at the JHU/APL (Johns Hopkins University /Applied Physics Laboratory) and now archived at the Space Physics Data Facility at NASA/GSFC ( Goddard Space Flight Center). 22)

• July 10, 2015: The TIMED mission is approved by NASA to continue planning against the current budget guidelines. Any changes to the guidelines will be handled through the budget formulation process. The TIMED mission will be invited to the 2017 Heliophysics Senior Review. 23)

- The TIMED spacecraft has been operational for more than a dozen years (launch Dec. 7, 2001) and data acquisition and processing appears to be autonomous. JHU/APL operates the TIMED Mission Operations on site and data are processed and archived in accordance with the TIMED Mission Archive Plan. Most of the labor is invested in processing and archiving SABER data. Science products from each of the instruments are produced on a continual basis and the plan is to transition all the data to the SPDF within 150 days after the end of the mission. 24)

- During its lifetime the TIMED mission has produced an important set of state variables (density, temperatures, dynamics, composition and energetics) of the mesosphere, thermosphere and ionosphere and has produced a number of discoveries in multiple areas. The mission has helped established connections between the lower and upper atmosphere in comparison to solar forcing. The low activity in SC-23 (Solar Cycle-23) and SC-24 has really helped make this possible. The TIMED science is providing important insights into the thermodynamics of the upper atmosphere and climate change.

• Feb. 2015: The TIMED spacecraft is operating nominally after more than 13 years on orbit and is currently collecting data on one of the last frontiers in Earth's atmosphere. 25)

- According to information from James Russell of Hampton University, the SABER instrument of TIMED is operating very well. SABER is an infrared limb sounder that measures profiles of T, O3, H2O, O, H and emission from NO, O2(1delta) and CO2 used for dynamics and energetics studies of the atmosphere.

• 2014: The TIMED spacecraft is operating nominally (in its 13th year on orbit) and is currently collecting data on one of the last frontiers in Earth's atmosphere. 26)

• The 2013 Senior Review revealed abundant evidence that Heliophysics extended missions are providing scientific value well beyond the realm of heliophysics. Measurements from the current constellation of extended missions support not only Heliophysics science objectives and the overarching goals of the SMD (Science Mission Directorate) but also specific goals of the other three divisions within SMD. 27)

- Heliophysics extended missions focussing on the physics and chemistry of the upper atmosphere — AIM (Aeronomy of Ice in the Mesosphere) and TIMED — are resolving the solar-terrestrial impacts on Earth's climate, which are larger than hitherto thought. The measurements provide a means of determining the impacts of energetic solar particles on the chemistry of the mesosphere and lower thermosphere, which affect stratospheric ozone and the circulation of the lower atmosphere. These results offer new insights into the sources of change in the Earth system, a primary science theme of NASA's Earth Science Division.

- TIMED, along with AIM, is the terrestrial anchor of NASA's HSO (Heliophysics System Observatory), and as such provides the majority of the "to Earth" link fo the "from Sun to Earth" connection. TIMED data provide for improved understanding of the fundamental processes of the space environment, which os the primary Heliophysics Research Objective.

- Spacecraft/instrument health status: The spacecraft and instruments are showing their age but appear to be capable of supporting the PSGs (Prioritized Science Goals) as planned. The SABER instrument appears to be in the best health and is the primary instrument for the extended mission. The GUVI instrument can no longer scan across the limb and is used in spectrographic mode at 30º off nadir. The TIDI performance has improved from what it was during the early orbit, but the PSGs do not strongly depend on it. The SEE instrument has only a 3% duty cycle (Ref. 27).

• In 2013, the TIMED spacecraft and its payload are operating nominally (in its 12 year on orbit). 28)

- In early March 2012, the SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) instrument on TIMED measured the impact of the huge solar flare (CME) on Earth's upper atmosphere. The upper atmosphere heated up, and huge spikes occurred in infrared emission from nitric oxide and carbon dioxide. 30)

• On Dec. 7, 2011, the TIMED team celebrated 10 years of on-orbit operations - collecting data over almost an entire solar cycle. - Since TIMED was launched, APL has been responsible for project science leadership, spacecraft mission operations and sustaining engineering, as well as public outreach and education initiatives. 31)32)

By studying daily, seasonal and yearly variations on a global scale, with the comprehensive data set collected by TIMED, the project is able to get a full picture of the driving forces behind the observed atmospheric changes.

• February 2011: The TIMED spacecraft is operating nominally and is currently collecting data on one of the last frontiers in Earth's atmosphere. 33)

• On October 1, 2010, NASA extended the TIMED mission through 2014. This is its fourth extension since the original 2-year mission began in January 2002. TIMED will focus this time on a problem that has long puzzled scientists: differentiating between human-induced and naturally occurring changes in this atmospheric region. This extension also allows TIMED to continue collecting data for longer than a full 11-year solar cycle. 34)35)

• The TIMED spacecraft and its sensor complement are "operating nominally" in June 2010. 36)

• The spacecraft and its sensor complement are operating nominally as of 2009 (the TIMED spacecraft is currently in its eighth year of operation, six years beyond the planned two year mission).

Over the course of this time degradation or outright failures have occurred with a star tracker, the gyros, a reaction wheel, and sun sensors. As a result, various changes have been made to the attitude estimation and control algorithms and autonomy rules to allow continued operation under these anomalous conditions. 37)

• In late 2009, the observations from TIMED show a dramatic cooling in the upper atmosphere that correlates with the declining activity of the current solar cycle. For the first time, researchers can show a timely link between the Sun and the climate of Earth's thermosphere, the region above 100 km, an essential step in making accurate predictions of climate change in the high atmosphere. This finding also correlates with a fundamental prediction of climate change theory that says the upper atmosphere will cool in response to increasing carbon dioxide. 38)

The TIMED measurements show a decrease in the amount of ultraviolet radiation emitted by the Sun. In addition, the amount of infrared radiation emitted from the upper atmosphere by nitric oxide molecules has decreased by nearly a factor of 10 since early 2002. These observations imply that the upper atmosphere has cooled substantially since then. The research team expects the atmosphere to heat up again as solar activity starts to pick up in the next year.

• NASA approved a third mission extension for completion in January 2013.

Figure 12: TIMED image of a major geomagnetic storm that occurred in April 2006 (image credit: JHU/APL)

Legend to Figure 12: The image of the geomagnetic storm is superimposed over an Earth image. Throughout its five years in operation, TIMED has served as a catalyst for a greater understanding of our thermosphere and ionosphere. In coordination with a network of space- and ground-based systems, TIMED has provided the first view of the mesosphere, ionosphere and thermosphere as a coupled system throughout a range of solar activity levels.

• In May 2006, NASA announced a second extension of the TIMED mission for 4 years with operations and data analysis continuing through 2010. 40)

• In January 2004, after the end of the nominal design life of 2 years, NASA extended the TIMED mission for another three years of operations and data analysis.

• In mid-2003, "first light" results from TIMED's suite of four instruments are presented. 41)

The excellent data set collected by UARS has given the MLTI research community a first glimpse at the MLTI dynamic structure. The project team has learned that its mean background winds have a clear seasonal structure and its daily global pattern is highly variable both spatially and temporally, dominated by the presence of tides and large-/small-scale waves.
Prior to TIMED, the full MLTI momentum balance had not been well characterized. Previously, global thermospheric density and composition information was based on in situ satellite drag and mass spectrometer data as well as ground-based incoherent radar data.2 Information on the mesosphere came from limited rocket measurements using mass spectrometers and falling spheres.

TIMED has provided the first simultaneous measurements of MLTI density, winds, and temperature structures, critical elements for a detailed understanding of MLTI momentum balance. It has also provided the first simultaneous measurements of MLTI energy inputs and radiative energy outputs that are needed to quantitatively understand the MLTI energy balance. Most importantly, TIMED observations allow the Sun-Earth Connection community, for the first time, to investigate the MLTI response to various types of external energy inputs.

• Science operations officially began on January 22, 2002 - about two months after launch.

Instrument PI: A. B. Christensen, The Aerospace Corporation. The instrument is of SSUSI heritage (flown on the DMSP F-16 S/C) and a joint collaboration between JHU/APL and The Aerospace Corporation, El Segundo, CA. The objective is to monitor three general regions: the daytime low- to mid-latitude thermosphere, the nighttime low- to mid-latitude ionosphere, and the high-latitude auroral zone. The goal is to obtain a detailed quantitative and predictive understanding of auroral phenomena. The instrument consists of the following elements: a scan mirror feeding a parabolic telescope and Rowland circle spectrograph, with a wedge-and-strip detector at the focal plane.

GUVI is a FUV scanning imaging spectrograph providing horizon-to-horizon images in five selectable bands or "colors." The colors chosen are: H 121.6 nm, O 130.4 nm, O 135.6 nm, and N2 Lyman-Birge-Hopefield (LBH) bands at 140 to 150 nm and 165 to 180 nm. GUVI uses a scan mirror to sweep its FOV of 11.78º through an arc of up to 140º (scan from 80º to -60º) in the cross-track direction. This FOV is mapped via an f/3 Rowland circle spectrograph (with a toroidal grating) into 14 spatial and 160 spectral "pixels." The scan sweep time is 22 s. The detector is a microchannel plate (MCP) intensified wedge-and-strip anode which provides a 2-D readout. 43)

The SIS (Scanning Imaging Spectrograph) of GUVI consists of a cross-track scanning mirror at the input to the telescope (a 75 mm focal-length off-axis parabola system with a 25 x 50 mm clear aperture) and a Rowland circle spectrograph. Two 2-D photon-counting detectors are located at the focal plane of the spectrograph. The operating detector is selected by a "pop-up" mirror that is moved into or out of the optical path to direct radiation from the grating onto one of two detectors. The detectors employ a position-sensitive anode to determine the photon event location.

The GUVI observations are compared with data collected by the ground stations for "ground truthing." GUVI products include maps of the auroral oval, the characteristic energy and flux of the electrons which excite it, F-region ionospheric electron density profiles, and dayside neutral composition information.

Parameter

H (121.6 nm)

O (130.4 nm)

O (135.6 nm)

N2 (LBH-1)

N2 (LBH-2)

Dayside limb

H profile and escape rate

O profile

O altitude profile

Solar EUV inputs

N2, O2, temperature

Dayside disk

H column

Composition

O/N2

O/N2

Nightside limb

H column

F-region electron density profile

Nightside disk

H column

F-region total electron content

Auroral zone

Proton auroral boundaries

Auroral boundaries

Effective energy flux Q

Effective average energy of precipitating particles

Table 3: GUVI environmental parameters measured by different colors

SABER (Sounding of the Atmosphere using Broadband Emission Radiometry):

Instrument PI: J. M. Russell, Hampton University; instrument builder: SDL of Utah State University, Logan, UT. SABER (heritage of LIMS, SAMS, CIRRIS, ATMOS, HALOE, CLAES, ISAMS, and SME) is a 10-channel radiometer with the objective to measure the IR emissions emitted at altitudes generally below 100 km by limb scanning to drive vertical distributions of temperature and concentrations of energetically important species (O3, H2O, NO, NO2 CO, CO2) as well as radiative energy loss. The telescope is an on-axis Cassegrain design (rejection of stray light outside IFOV). The telescope and baffle assembly are cooled to 240 K by a dedicated radiator. The focal plane assembly, consisting of a filter array, a detector array, and a Lyot stop, is cooled to 75 K by a miniature cryogenic refrigerator. Instrument drifts due to changes in telescope and focal plane base temperatures are corrected by an in-flight calibration system. The detector arrays are: five HgCdTe (photoconductive mode of operation), two InSB (photovoltaic mode of operation), and three InGaAs (photovoltaic mode of operation). Instrument mass=61.7 kg, power = 64 W, the data rate is 4 kbit/s. 44)45)

Spectral coverage: discrete ranges from 1.27 - 17 µm (7865 cm-1 to 650 cm-1). A scan depressing angle of -20.0º to -13.9º from the S/C horizontal is used; vertical resolution of 2 km. SABER scans up and down the Earth's horizon once every 58 seconds, collecting data over an altitude range from about 180 km down to the Earth's surface (vertical profiles of elemental constituents) and temperature. Based on the thermal emission characteristics of its measured CO2 15.4 µm emission, SABER provides atmospheric temperature measurements critically needed to study atmospheric momentum and energy balance. In addition, SABER measures the vertical distributions of molecular constituents (e.g., ozone, water vapor) that are important for their direct role in solar photon energy absorption and their indirect role in chemical reactions involving energetically important chemical species.

SABER is the first spaceborne instrument to provide systematic global measurements of atmospheric density and temperature as well as key atmospheric heating and cooling rates.

SEE (Solar EUV Experiment):

Instrument PI: T. N. Woods; SEE was built by LASP (Laboratory for Atmospheric and Space Physics) of the University of Colorado, Boulder, CO. The solar sensors in SEE are EGS (EUV Grating Spectrometer) and XPS (XuV Photometer System). The objective is to measure the absolute fluxes of solar UV, EUV, and XUV radiation and to determine the rates of energy deposition, dissociation and ionization. 46)

XPS consists of a set of nine XUV silicon photodiodes (thin film coatings on diodes) as detectors, designed to measure the full-disk solar soft X-ray spectral irradiance at several fixed spectral wavelengths. It provides solar irradiance measurements from 0.1 to 35 nm, with each photometer having a spectral bandpass of 5 to 10 nm. FOV = 20º in diameter. An additional filtered photometer is a bare X-ray ultraviolet photodiode with Acton Lyman-alpha filters for a redundant measurement of the important Lyman-alpha irradiance.

Instrument PI: T. L. Killeen, UCAR (University Corporation for Atmospheric Research), Boulder, CO. Objective: investigation of the dynamics and energetics of MLTI. TIDI measures the VIS/NIR emissions emitted at altitudes between 60 and 300 km by limb scanning techniques to determine the temperature and horizontal winds with the use of the Doppler effect. The instrument makes also density measurements, mostly on the day side of the orbit.

TIDI comprises three subsystems: four identical telescopes (off-axis Gregorian type, aperture 7.5 cm, f/2.2 FOV = 2.5º horizontal x 0.05º vertical), a Fabry-Perot interferometer (fixed-gap single etalon) with a CCD detector (passively cooled), and an electronics box. TIDI views emissions from OI 557.7 nm, OI 630.0 nm, OI 732.0 nm, O2 (0-0), O2 (0-1), Na D, OI 844.6 nm, and OH to determine Doppler wind and brightness temperature of the atmospheric airglow emission lines at a very high spectral resolution throughout the altitude range. TIDI obtains these scans simultaneously in four orthogonal, azimuthal directions: two at 45º forward but on either side of the spacecraft's velocity vector and two at 45º rearward of the spacecraft. 48)49)

Figure 19: Illustration of the TIDI telescope (image credit: UCAR)

Each vertical scan consists of individual views of the limb at 2.5º (horizontal, along the limb) x 0.05º (vertical, normal to the limb) angular resolution or at 125.0 km x 2.5 km spatial resolution. The altitude step size ranges from 2.5 km in the mesosphere to 25.0 km in the thermosphere. Each up/down acquisition cycle takes about 100-200 s to complete, resulting in a nominal horizontal spacing between profiles of approximately 750 km along the orbit track.

The TIMED operations system includes the MOC (Mission Operations Center), MDC (Mission Data Center), and the SCF (Satellite Communications Facility) at APL, four POCs (Payload Operations Centers) located at facilities across the country, and the distributed SDS (Science Data System). 50)

Mission operations and instrument data distribution are the responsibility of the Laboratory (APL). These functions are performed at the TIMED Mission Operations Center (MOC) on the APL campus. The instrument teams operate their instruments directly from the POCs, located at the institutions where the instruments were developed.

The TIMED spacecraft was designed with a high degree of autonomy to enable inexpensive mission operations using a small Mission Operations Team consisting of eight members. One key to making this possible is a decoupled instrument operations approach. TIMED is the first APL mission to be operated in this manner. The strategy is based on the concept that the spacecraft is the "bus" and the instruments are the "passengers."

The decoupled operations approach is further illustrated in Figure 22. The mission data flow is in two paths. The outer path, indicated in green, represents the science data flow of commands and telemetry and shows the principal activities of the POCs. All of the processes in the spacecraft, as well as the mission operations ground system portion of this path, are automated. The inner path of bus activity is the mission operations data flow, which is generated and processed independent of the science data flow. Thus the Mission Operations Team operates the spacecraft bus while the instrument teams operate their instruments. No personnel effort is expended to merge the two sets of activities, thereby saving a great deal of operational costs.

Legend to Figure 22: The outer path (green) shows the flow of instrument data, both commands and telemetry. The inner path shows the flow of spacecraft bus commands and telemetry. The left side represents spacecraft processes, while the right side represents ground processes.

Mission operations: Unlike previous spacecraft ground systems, the TIMED ground system design was not driven by the spacecraft design. Instead, the end-to-end system design for both spacecraft and ground was driven by the desire to reduce operational costs by easing spacecraft and instrument operations. This led to a highly autonomous system that uses distributed processes communicating over local and wide area networks. Figure 23 is a diagram of the major components of the TIMED ground system.

The TIMED MOC is located on the APL campus near the SCF (Satellite Communications Facility). The MOC houses the computer systems used to operate the spacecraft, process and store the mission data, and serve the science community.

The MDC (Mission Data Center) was developed to support the TIMED mission concept of operations in which geographically dispersed POCs operate their instruments remotely and independent of spacecraft bus operations. To support this approach, a data system was developed that provides completely automated online data archival and delivery functionality. The MDC provides the POCs and MOC with several essential services including:

• The capability for real-time monitoring of spacecraft and instrument telemetry during ground station passes

• On-demand playback of spacecraft and instrument data

• Short- and long-term data archival

• The production of supporting mission data products.

The Router and the Archive Server are the two main software components that make up the MDC's Telemetry Server. These workhorses provide the data delivery and archival functionality.

SDS (Science Data System): An important goal of the TIMED mission is to quickly create and disseminate processed atmospheric science data to the scientific community, K–12 educators, and the general public in addition to the TIMED program elements. The objective is to produce an initial version of routine science products, available to all TIMED users, within 54 hours after telemetry acquisition on the ground.

The TIMED ground system includes a distributed SDS. The SDS is composed of the TIMED MDC and those portions of the POCs involved with the processing and distribution of science data products (Figure 24). As in the typical space mission science center, the SDS is responsible for the acquisition, generation, distribution, and archiving of science data necessary to support the TIMED mission. Unlike a traditional mission science center; however, these functions of the SDS are distributed over its component facilities. Supporting its goal of disseminating science products, the SDS uses a Web site as its common user interface and relies on standard protocols of FTP and Web document transfers across the Internet.

Legend to Figure 24: The TIMED SDS includes the data archive and distribution functions of the MDC as well as the science data processing portion of the POCs. It provides the data interface to other external collaborators and users. Blue lines represent MDC and POC data input; red lines represent their data output.

The information compiled and edited in this article was provided byHerbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).